The present invention relates to a method and apparatus for forming a thin film by a CVD method and a method of manufacturing a semiconductor device comprising the step of forming a thin film by a CVD method.
With progress in the degree of integration of a DRAM, it is of high importance to decrease the thickness of a capacitor insulating film. In the case of using the conventional material for forming a capacitor insulating film, the capacitor insulating film must be formed very thin in order to obtain a sufficient electrostatic capacitance. Therefore, it is proposed in recent years to use a material having a high dielectric constant such as BaSrTi oxide (hereinafter referred to as "BSTO") for forming a capacitor insulating film.
In forming a BSTO thin film on a semiconductor substrate by a CVD method, it is ideal to use as a Ba source and a Sr source materials having a high vapor pressure and gaseous at room temperature. However, such a Ba source and a Sr source are unknown. Therefore, materials solid at room temperature and sublimated to form a gaseous phase when heated to a predetermined temperature are used as the Ba source and Sr source, said materials including, for example, bis(2,2,6,6-tetramethyl-3,5-heptanedionate)barium (hereinafter referred to as "Ba(THD).sub.2 "), and bis(2,2,6,6-tetramethyl-3,5-heptanedionate)strontium (hereinafter referred to as "Sr(THD).sub.2 "). Also, bis(2,2,6,6-tetramethyl-3,5-heptanedionate)titanium oxide (hereinafter referred to as "TiO(THD).sub.2 ") was used as a Ti source. Incidentally, TiO(THD).sub.2 is solid at room temperature and sublimated when heated.
FIG. 1 schematically shows a conventional CVD apparatus used for forming a BSTO thin film. In the conventional method, a solid Ba(THD).sub.2 16 housed in a container 7 is heated by a heater 25 to 215.degree. C. so as to be sublimated to generate a Ba(THD).sub.2 gas. A nitrogen gas is supplied as a carrier gas from a nitrogen gas supply source 14 into the container 7. The flow rate of the nitrogen gas into the container 7 is controlled by a mass flow controller 22. It follows that the Ba(THD).sub.2 gas and the nitrogen gas within the container 7 are supplied together into a chamber 1 through a valve 6.
A Sr(THD).sub.2 gas and a TiO(THD).sub.2 gas are also supplied similarly into the chamber 1. To be more specific, a solid Sr(THD).sub.2 17 housed in a container 20 and a solid TiO(THD).sub.2 18 housed in a container 19 are heated to 215.degree. C. and 130.degree. C., respectively, so as to generate a Sr(THD).sub.2 gas and a TiO(THD).sub.2 gas. Also, a nitrogen gas is supplied from a nitrogen gas source 14 into the containers 20 and 19 through mass flow controllers 21, 23, respectively. Naturally, the Sr(THD).sub.2 gas and the TiO(THD).sub.2 gas are supplied together with the nitrogen gas from the containers 20 and 19 into the chamber 1 through valves. Further, an oxygen gas is supplied from an oxygen gas supply source 15 into the chamber 1 through a mass flow controller 24.
The Ba(THD).sub.2 gas, Sr(THD).sub.2 gas, TiO(THD).sub.2 gas, oxygen gas and nitrogen gas are mixed within the chamber 1 to form a gas stream 5 within the chamber 1. The gas pressure within the chamber 1 is monitored by a pressure gauge 13 and controlled at about 10 Torr by a conductance valve 12 for pressure control.
A wafer 2 and a susceptor 8 within the chamber 1 are heated to about 600.degree. C. by the light emitted from a lamp 3 and transmitted through the quartz wall of the chamber 1. As a result, the mixed gas within the chamber 1 is partially decomposed, and the decomposed materials carry out reactions. The reaction product is deposited on the wafer 2 to form a BSTO thin film.
A stagnant layer 4 through which a gas does not flow is formed in the vicinity of the surface of the wafer 2 during formation of the BSTO thin film. The mixed gas forming the gas stream 5 is partly supplied into the stagnant layer 4. The mixed gas supplied into the stagnant layer 4 is diffused within the stagnant layer 4 so as to reach the wafer surface. As a result, the raw material gas components contained in the mixed gas are decomposed so as to bring about deposition of BSTO.
As described above, the stagnant layer 4 contributes to the deposition of BSTO. Therefore, in order to form a BSTO thin film of a uniform thickness, it is necessary to control highly accurately the thickness, etc. of the stagnant layer 4 and, thus, to make the deposition rate uniform.
However, the thickness of the stagnant layer 4 tends to be affected by the gas stream 5. Also, it is very difficult to keep the gas stream 5 constant and uniform, leading to a non-uniform supply of the raw material gas components onto the wafer surface and to non-uniform deposition rate. It follows that it is difficult to form a BSTO thin film of a uniform thickness.
It should also be noted that the amount of the raw material gas components supplied from the gas stream 5 into the stagnant layer 4 is dependent in general on the partial pressure of the raw material gas components contained in the mixed gas forming the gas stream 5. Under the conditions described above, the amount of the raw material gas components supplied into the stagnant layer 4 is only several percent of the raw material gas components contained in the mixed gas forming the gas stream 5. In other words, the amount of the raw material gas components which are decomposed and consumed for the formation of the BSTO thin film is only several percent of all the raw material gas components supplied to the chamber 1. Naturally, a major portion of the raw material gas components supplied to the chamber 1 is not decomposed so as to be discharged to the outside of the apparatus through a main valve 9, a conductance valve 12 for the pressure control, a pipe 11 and a pump 10, leading to a markedly high manufacturing cost of a semiconductor device including a BSTO thin film.
It should also be noted that the Ba(THD).sub.2 gas, Sr(THD).sub.2 gas and TiO(THD).sub.2 gas used in the conventional method are prepared by sublimation of the solid raw materials 16 to 18. This makes it necessary to heat the pipe, valve, etc. connected to the chamber 1 so as to prevent the Ba(THD).sub.2 gas, etc. from being solidified. However, the valve, etc. used in the CVD apparatus tends to bring about deterioration of the driving section when the valve is exposed to high temperatures. It follows that the valve, etc. must be renewed frequently.
In order to suppress the deterioration, it is necessary to set the heating temperature of the pipe, valve, etc. at a low level. If the heating temperature is lowered, however, the flow rate of the raw material gas components must be maintained at a low level in order to prevent the raw material gas components from being solidified.